Protective casings of this type ar already known in the art, U.S. Pat. No. 4,435,166 discloses a protective casing. This protective casing also has two elements or cowlings that cover the joints on the shaft, which are connected by the system of telescopic tubes. Thus this type of protective casing, and other similar types, cover the entire transmission shaft formed by two universal joints and two tubular elements that form a telescopic connection. In order to ensure the transmission shaft functions well and is not subject to excessive wear, the tubular elements require lubricating from time to time. This task requires uncoupling the transmission shaft and removing the two tubular elements so that lubricant can be applied. This amounts to a time consuming and difficult task, which is often carried out less frequently than is necessary to lubricate the shaft well thus ensuring it operates correctly and is not subject to excessive wear.